If you think a lect which Wiktionary covers is not an independent language but only a dialect of another language which should be covered under the latter's headers and categories, or if you think a lect should be de-recognized for some other reason (e.g. because it is spurious):

Update/Eliminate as many uses of the language's name and code as you can find: look through the language's categories, update the entries in them and delete the categories themselves; search the site for instances of the language's name to find entries in translations tables; search a database dump if you know how.